Numerous variants of devices of this type are conventionally known. For example, devices have conventionally been provided with temperature sensitive markings in the form of bar codes, which make it possible to transfer a wide variety of information concerning not only a heating program and temperature conditions, but also previous temperature related handling of packages with food.
The known devices have proved to work sufficiently with packages containing fluid food, where the read temperature on the surface of the package closely follows the temperature of the contents of the package. Packages containing predominantly solid food, however, are subject to a considerable delay with respect to the determined temperature on the surface of the package compared to the temperature within the food. There are also other irregularities depending on the kind of food in the package.